Point spectroscopy and spectral imaging of target tissue have been used to assess the condition of tissue in a patient, for example the presence of various illnesses or diseases. Such spectroscopy and imaging have used a variety of different techniques to effect such assessments, including absorbance spectroscopy in transmission and reflectance modes, fluorescence spectroscopy, and Raman spectroscopy (see, e.g., U.S. Pat. No. 4,836,203; U.S. Pat. No. 5,042,494; U.S. Pat. No. 5,062,428; U.S. Pat. No. 5,071,416; U.S. Pat. No. 5,421,337; U.S. Pat. No. 5,467,767; U.S. Pat. No. 5,507,287; Mahavedan-Jansen, A. and Richards-Kortum, R., J. Biomed. Optics I(I):31-70 1996; U.S. Pat. No. 5,261,410). These techniques have been used in both single and multi-point measurements, and have been used in array spectroscopy to generate images on an imaging device such as a television screen.
A common form of array spectroscopy comprises the use of a color video camera in which a lens system projects an image through optical filters that select a desirable wavelength band or sub-region of the light forming the image, and then projects the image onto an array of sensors. The sensors convert the light into an electrical signal that is proportional to the light incident on a given sensor. The electrical signal can then be stored in a digital or analog format, and/or relayed directly to a device that displays the image, such as a color video monitor. This sort of a system where an image is captured by sensors, converted to electrical or other signals, and then re-created on a viewer (i.e., transduction of the original optical image to a different type of energy and then creation of a new image corresponding to the original image) is known as an imaging device or imager.
The color display monitor recreates an image viewable by a user by converting the electrical signals into optical signals. In this process, a color phosphor that emits light in a narrow wavelength region is stimulated by an amount proportional to the signal originally collected by its corresponding sensor. A conventional color monitor has red, green and blue phosphors. The wavelengths of light emitted by the phosphors of the monitor do not necessarily need to match the wavelengths of light that were collected by the sensors for the color to be perceived by the viewer as "normal." In particular, the photoreceptors of the human eye respond to only three wavelength regions, and colors perceived by humans only require that the eye be stimulated somewhere in a given wavelength response range for each of the red, green and blue photoreceptors of the eye.
Indeed, using such an imaging device, the colors received by the individual viewing the monitor do not necessarily have to bear any relationship to the wavelength of light originally perceived by the sensor. This property is often exploited with such devices as infrared cameras or x-ray or gamma cameras, where the sensors detect radiation that is well outside of the human visible range and then map the detected radiation to colors in the visible range. This use of imaging devices has also been applied to medical fluorescence imaging systems for detecting changes in the light emitted by tissue that has been illuminated with blue or ultraviolet light (or other light that induces a desired response). Imaging devices have been developed where the image sensors detect particular wavelength sub-ranges or bands of fluorescent light, process and scale the electronic signal, and then display the signal in two colors on a video monitor.
The methods and apparatus employed in using such imaging devices as described above require bulky systems including computers, television monitors and expensive sensors, analyzers and computer software and hardware. Such systems often also have a limited dynamic range. Accordingly, there has gone unmet a need for apparatus and methods that reduce the expense, bulkiness and/or complexity of, and increases the range for, viewing a target tissue. The present invention provides these and other related advantages.